Submerged dam is a commonly used structure to control the reaches of a braided river, and it is generally located at the entrance of the inlet. The flows in these areas are often complex. It may cause downstream scouring after the completion of the submerged dam and the consequent uneven subsidence. Riprap reinforcement is an important measure of maintaining the function of the submerged dam. The conventional riprap reinforcement project is used in the bank protection. The traditional stone-throwing distance formulas are generally applied to shallow water and related to the water surface velocity, water depth, and the weight of the stones. There is not much study on the riprap reinforcement project in deep water area, and the research on the stone scattering is much less. In this paper, the authors make the first attempt to conduct-field tests for investigating the throwing distance and the enlargement coefficient, and for optimizing the comprehensive scattering coefficient. These field tests were performed at the entrance of the left branch of Hechang-bar, which is located at the lower reach of the Yangtze River, China. Based on the vertical distribution of velocity, the formula of the fall velocity of stone in deep water area, the formula of the throwing distance and the formula of the comprehensive discrete coefficient are derived. These formulas are verified by field data. The research extends the traditional throwing distance formula to the deep water area, and discusses the dispersion degree of throwing stones. The results can provide reference for submerged dam reinforcement works or other similar construction projects.